Beaconless communication system

ABSTRACT

The present invention relates to an energy efficient mechanism for establishing and maintaining a communication between nodes (A . . . F) in a wireless communication system. In particular, it relates to a distributed low power medium access control (MAC) mechanism for sharing the communication means in a wireless communication system. According to the present invention a destination node (A) listens to a communication channel periodically. For requesting services from the destination node (A), a wake-up signal (WU) is transmitted from a source node (B) to the destination node (A) via the communication channel, wherein the wake-up signal (WU) is formed by a preamble (WP) and an information for controlling the communication between nodes (A . . . F). The WU signal duration and content is adapted depending on the system operation context to reduce either idle times, power consumption, latency or network blocking (collision avoidance).

The present invention relates to an energy efficient mechanism forestablishing a communication between nodes in a communication system. Inparticular, it relates to a distributed low power medium access control(MAC) mechanism for sharing the communication means in a wirelesscommunication system.

In a wireless mesh/ad-hoc network each device (node) can communicatewith any other device within its transmission range. If source anddestination nodes of the network are not within the transmission rangeof each other, a message can be passed through one or more intermediatenodes until the message reaches the destination node. Thus, the coveragearea of a mesh/ad-hoc network expands naturally as the number ofnodes/users increases. Furthermore this type of networks is veryresilient to operation failures happening in individual nodes, becausethe network is able to find another path for the messages avoiding thedefect nodes.

The nature of the mesh/ad-hoc networks implies that the access to theradio resource has to be done in a distributed manner. A simpledistributed access mechanism is the Carrier-Sense MultipleAccess/Collision Avoidance (CSMA-CA) which is a radio access schemewhere the occupancy of the carrier is measured and detected beforeutilisation of the medium to reduce the probability of collisions andsubsequent data loss and/or need for data retransmission. This mechanismis a valid option and widely used in standard wireless Local AreaNetworks (LANs) like IEEE 802.11x, IEEE 802.15.3 or IEEE 802.15.4.However, due to the lack of a central controller which provides networktiming synchronisation, the peer nodes in a mesh/ad-hoc network mustfind an alternative distributed solution to get each othertime-synchronised and be able to exchange information because thedestination node must be in reception mode when the source node istransmitting.

The easiest solution is that the destination nodes keep their receiverson (activated) all the time, which ensures that every transmission willbe received if no collision occurs. Of course this solution is notapplicable to small battery powered nodes (portable devices) due topower consumption reasons.

How to reduce the power consumed by the hardware radio in wirelessdevices has been an important research topic in the last few years.Several solutions have been proposed like adjusting the transmissionpower just to reach the destination node. Further, in Guo, C., et al.,“Low Power Distributed MAC for Ad Hoc Sensor Radio Networks,” IEEEGlobeCom 2001, November 2001, is suggested to use multi-hopping throughintermediate nodes, or a specific hardware unit which allows waking-up asleeping node remotely.

Nevertheless the most effective way to reduce the power consumed by thehardware radio (transceiver system) in a portable device of for examplea low traffic network is to switch-off the radio whenever the radio isnot being used for either data transmission or reception.

However, the problem to solve is how to synchronise the transmission andreception times. A power efficient solution is to switch on the receiveronly at certain times, and use small data packets (beacons) which aretransmitted periodically by every node, to inform other nodes in theneighbourhood about its listening schedule, i.e. when and for how longits receiver will be switched on. A beacon based MAC solution isproposed in IEEE 802.11 for ad-hoc operation in an Independent BasicService Set (IBSS).

A beaconless solution for a low traffic network is described inEl-Hoiydi, A., et al., “Low Power MAC Protocols for InfrastructureWireless Sensor Networks”, European Wireless 2004, Barcelona, 24-27February 2004. There is proposed a MAC protocol called “WiseMAC” bywhich the receivers are activated with the same constant period andlisten to the radio channel for a very short period of time, just longenough to be able to receive one modulation symbol. In the WiseMACprotocol, instead of using beacons, the source node transmits, in frontof every data, a wake-up preamble of a duration which is equal to themedium sampling period. All nodes which sense/sample the mediumperiodically and receive the wake-up signal continue to listen until adata packet is received. Further, for reducing the duration of thewake-up signal and for exact matching the listen time of a predetermineddestination node, it is proposed that every node should learn thesampling schedule of all nodes. However, in the WiseMAC protocol, thereis an energy waste since it is not always necessary to activate thereceiver, if a wake-up signal is received by a node.

It is an object of the present invention to provide a communicationmethod, a communication system, an apparatus for requesting servicesfrom a remote apparatus and an apparatus for providing servicesrequested from a remote apparatus which reduce the power consumption ina communication system. This object is achieved by means of features ofthe enclosed independent claims. Advantageous features of the presentinvention are defined in the corresponding subclaims.

According to the present invention the destination node listens to acommunication channel periodically, wherein the listen duration is T_(L)and the listen periodicity is T_(PL) and T_(L)<<T_(PL). For requestingservices from the destination node, a wake-up signal is transmitted froma source node to the destination node via the communication channel,wherein the wake-up signal comprises a preamble and an information forcontrolling the communication between nodes. The duration of a signalblock formed by the preamble and the control information is T_(SB) andT_(SB)<T_(L), preferably: 2T_(SB)<T_(L). Thus, with the presentinvention information like an address of a predetermined node or smallcommands which do not need additional data can be added to the wake-upsignal. In this way the activation of the receiver can be controlledmore exactly, i.e. whether and when the receiver has to be switched on.Further, the communication between nodes can be optimized and the powerconsumption reduced without additional data transmissions like beacons.

Preferably, the control information indicates a time point at which thesource node starts a data transmission so that the destination nodereceiving the control information can switch off its receiver until thedata transmission starts to save power.

Further, when the communication system comprises a plurality of nodesforming a network and the control information includes addressinformation, a predetermined node can be addressed, wherein all nodeswhich receive the wake-up signal and are not addressed do not listen tothe communication channel and/or transmit any data via the communicationchannel for a predetermined period of time. Thus, the power consumptionis reduced and no collision occurs. The combination of addressing andtime reference in the wake-up signal facilitates broadcast and multicastscenarios.

Advantageously, the control information indicates the duration of asubsequent data transmission. This information could be used as aNetwork Allocation Vector (NAV) by other nodes which detect when themedium is free, before starting the transmission of a wake-up signal.

For compensating drifts of the listen timing in the destination nodeand/or for connecting a plurality of destination nodes by a singlewake-up signal, the wake-up signal comprises a repetition (i.e. a numberof copies) of the signal block formed by the preamble and the controlinformation, wherein listen duration T_(L) is at least twice theduration of the signal block T_(SB).

Further advantageously, in the control information, the command forselecting, on the receiving side, a predetermined communication channelcan be included; a “Network joining” message can be used by a new nodeto indicate that it wants to join the network; or an “I'm alive” messagecan be used by a node which is already part of the network to informother nodes within its transmission range that it is still alive orpresent, wherein this message may request or not an acknowledgement fromthe receiver node(s).

Preferably, to minimize the wake-up signal duration, the nodesinterchange information of listen timing, and each node accumulates thetiming information, wherein the source node calculates the start of thewake-up signal based on the timing information.

Further preferably, each node calculates clock drifts with respect toany other peer node based on the received timing information, whereinthe source node calculates the start and/or the duration of the wake-upsignal based on the drift. Thus every node learns about the real clockdrift of its neighbour nodes, and also shares the accumulated timinginformation with other nodes to speed up the learning process for thenew nodes joining the network.

It should be emphasised that the term “comprises/comprising” when usedin this specification is taken to specify the presence of statedfeatures, integers, steps or components but does not preclude thepresence or addition of one or more other features, integers, steps,components or groups thereof.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 schematically shows a wireless communication network including aplurality of nodes employing an embodiment of the present invention,

FIG. 2 shows a timing diagram illustrating an example of the wake-upmechanism with respect to one node according the present invention,

FIG. 3 shows a timing diagram illustrating a further example of thewake-up mechanism with respect to one node according the presentinvention,

FIG. 4 shows a timing diagram illustrating an example of the wake-upmechanism with respect to a plurality of nodes according the presentinvention,

FIG. 5 shows a timing diagram illustrating an example of the wake-upmechanism and the interchange of data and listen timing informationaccording the present invention,

FIG. 6 illustrates, in a timing diagram, an example of the clock driftcalculation according the present invention, and

FIG. 7 shows a block diagram of an example of a wireless node shown inFIG. 1.

FIG. 1 schematically shows a wireless communication network 1 includinga plurality of nodes A . . . F in which each node A . . . F cancommunicate with any other node A . . . F within its transmission range.The nodes A . . . F of the network 1 may be sensors, e.g. smokedetectors, fire alarm sensors, but also other kinds of devices such asactuators, personal digital assistants (PDA), mobile phones, etc.

According to the present invention, instead of periodically transmittinga beacon, the nodes A . . . F in the network 1 implementing the MACprotocol activate their receivers (not shown) and listen to the physicalmedium (air interface) for a very short period of time compared with theperiod of time in which the receivers are switched off. The listenperiodicity T_(PL) as well as the listen duration T_(L), calledreception slot are common to all the nodes A . . . F of the network 1,wherein T_(L)<<T_(PL). When a node detects the wake-up signal, thetransmitter and receiver nodes can start the synchronisation phase toexchange data.

FIG. 2 shows a timing diagram illustrating an example of the wake-upmechanism in which the node B sends a wake-up signal WU to remotelyactivate the destination node A, wherein the node B wants to send datato the destination node A via a communication channel. As shown in FIG.2, the reception slots RX of the node A have the duration T_(L) and theperiod T_(PL). The node B sends the wake-up signal WU having a durationT_(WU) to inform the destination node A to keep its receiver activatedfor the subsequent data transmission following the wake-up signal WU. Ifthe destination node A is within the transmission range of the sourcenode B, and the wake-up signal duration T_(WU) is long enough, i.e.T_(WU)=T_(PL), one of reception slot TX of the destination node A willreceive the wake-up signal WU.

As shown in FIG. 3, the wake-up signal WU preferably comprises arepetition of a signal block SB formed by a preamble WP and a small datapacket WD. The preamble WP and the data packet WD are called “wake-upwhistle” and “wake-up data” respectively. To ensure that at least onecomplete signal block SB can be received during any reception slot RX,the duration T_(SB) of the signal block SB (whistle+data) must be lessor equal than one half of the reception slot duration T_(PL).

The node B transmitting the wake-up signal WU can embed differentmessages in the data packet WD, e.g. a command for selecting, on thereceiving side, a predetermined communication channel, a “Networkjoining” message which indicates that the node B wants to join thenetwork 1, or a “I'm alive” message which indicates that the node B isstill alive or present in the network 1, wherein this message mayrequest or not an acknowledgement from the receiver node A, and/or a“MAC data” message which indicates that the node B has user data totransmit, and that the transmission will start almost immediately afterthe wake-up signal. This is illustrated in FIG. 3, where a time pointerwhich indicates the data transmission starting point is inserted in thedata packet WD. The time pointer is used by the node A to synchroniseits receiver with the transmitter of node B.

Further, the destination node address can also be inserted in thewake-up data packet WD. If a physical address is available in thenetwork 1 or the MAC address is not too long, the destination addresscan be a part of the wake-up data packet WD. The benefit would be thatonly the destination node A would activate its receiver at the timeindicated by the time pointer. Any other node A . . . F catching thewake-up signal WU would not activate its receiver to decode the userdata, which saves power in the nodes A . . . F not intended to receivethis message. Broadcast or multicast addresses could also be used forsending information to multiple reception nodes A . . . F efficiently.

There are two important aspects regarding the time pointer. The firstone is that the time pointer does not represent or indicate an absolutetime value, but a relative one, as it is difficult to provide the sameabsolute time in the destination node(s). The time pointer indicateswhen the data transmission will start, but as a value which is measuredfrom now on. This means that the time pointer of each wake-up datapacket WD of the plurality of wake-up data packets WD in the wake-upsignal WU must have a different value, which is calculated by the node Bthat is transmitting the wake-up signal WU. I.e., in the successivesignal blocks SB of the wake-up signal WU, the length of the successivetime pointers decreases.

The reason the time pointer indicates the actual instant every time isthat the start of the wake-up signal could not be used because thereceiver node A does not know when that wake-up signal WU started. Onthe other hand the end of the wake-up signal WU could be used asreference point for the time pointer, but this solution wastes power.

As the time pointer is not referred to the end of the wake-up signal,the receiving node A does not need to keep its receiver active duringthe rest of the wake-up signal WU. On the contrary, the node A justreceives and decodes the wake-up data, and uses the embedded timepointer information to switch on the receiver at the right time.Obviously this reduces the time in which the receiver of the destinationnode A is activated, and hence more power is saved.

The benefit provided by the time pointer is even higher when a multicastor broadcast case is considered. In such a scenario only one wake-upsignal WU has to be transmitted. The duration T_(WU) of the wake-upsignal WU will be the maximum value for a broadcast transmission, or ashorter value calculated to catch up a group of nodes A . . . F in amulticast case. In any case, as shown in FIG. 4, all implicated nodes A,C and D will switch on their receivers only when the data transmissionstarts. Thus, the nodes A, C and D do not need keep their receiversactive to detect the end of the wake-up signal WU to use it as timereference point.

Together with the time pointer it is also possible to transmitinformation about the duration of the subsequent data transmission. Thisinformation could be used as a Network Allocation Vector (NAV) by othernodes A . . . F which detect when the medium is free, before startingthe transmission of a wake-up signal WU.

In the embodiment described above, there is a clear drawback, which isthe duration T_(WU) of the wake-up signal WU. A long wake-up signal WUnot only consumes power on the transmitting side but also occupies theradio channel for a substantial period of time, which prevents othernodes A . . . F to send other wake-up signals WU or may cause collisionswith other ongoing transmissions (hidden node scenario).

According to a further embodiment of the present invention, for reducingthe duration T_(WU) of the wake-up signal WU, while still ensuring thecatching of the reception slot RX of the destination node A, the nodes A. . . F in the network 1 keep a table where they store information aboutthe local time when the last communication with other nodes A . . . Ftook place. During a communication between two nodes A . . . F everynode A . . . F piggybacks, as part of its data or acknowledgement, thetime T from now on, when its next reception RX slot will take place.Furthermore, each node which receives the time information T alsostores, in the same table, the local time t when the last bit of thelast packet was received.

The Table 1 shows an example of a timing table stored in an internalmemory of a node #2 having a plurality of peer nodes #1, #3 . . . #n.TABLE 1 Node last communication time time to the next reception slotNode #1 t₁ T₁ Node #3 t₃ T₃ . . . . . . . . . Node #n t_(n) T_(n)

FIG. 5 shows a timing diagram illustrating an example of the wake-upmechanism and the interchange of data and the listen timing informationwith respect to the node #1 and node #2. As shown in FIG. 5, the node #2transmits the time T₂ along the data, wherein the node #1 detects thetime t₂ and records the timing information t₂, T₂ of the node #2. On theother hand the node #1 transmits the time T₁ along the data oracknowledgement, wherein the node #2 detects the time t₁ and records thetiming information t₁, T₁ of the node #1.

When the node #2 wants to establish a communication with the node #1,the node #2 reads the timing information t₁, T₁ of the node #1 from thetable and calculates the time t_(NXT) of the next reception slot RX ofthe node #1 based on the timing information t₁, T₁. The time t_(NXT) ofthe next reception slot RX of the node #1 is calculated by:t _(NXT) =t ₁ +T ₁+(N+1)T _(PL)   (1)wherein N is the number of listening periods T_(PL) (receiving slots)since t₁ and T_(PL) is the listen periodicity, which is common to allthe nodes #1 . . . #n. The number N of the listening periods T_(PL) iscalculated by: $\begin{matrix}{N = {E\lbrack \frac{t_{NOW} - t_{1} - T_{1}}{T_{PL}} \rbrack}} & (2)\end{matrix}$wherein E is the entire part operator.

As shown in FIG. 6, due to the clock drifts between the system clocks ofthe nodes #1 and #2, the starting time t_(NXT) calculated by equation(1) differs from the real starting time of the next reception slot RX ofthe node #1. Thus, if the wake-up signal WU of the node #2 shall bematched exactly to the reception slot RX of the node #1, the clockdrifts must be taken in to account when calculating the starting timet_(NXT) and the duration of the wake-up signal T_(WU).

The clock drift between the system clocks of the nodes #1 . . . #n of anetwork 1 is typically specified in parts per million (ppm). Forexample, a clock with a drift of 5 ppm may show a maximum deviation fromthe exact value of +5 μs or −51 μs every second. Therefore the absoluteclock drifts in both nodes #1 and #2 are proportional to the elapsedtime since they had the last communication, which is indicated by thetime t₁.

The maximum clock drift time T_(ADD) occurred in each of the nodes #1and #2 since the last communication is calculated by:T _(ADD)=θ(t _(NXT) −t ₁)=θ(T ₁+(N+1)T _(PL))   (3)wherein θ is the clock drift. However, the node #2 should start thetransmission of the wake-up signal WU two times the maximum clock driftT_(ADD) before the calculated t_(NXT). The reason for this is the worstcase scenario in which one clock of the nodes #1, #2 could be advancedwhile the other delayed the same amount. The calculated starting pointt_(WU) _(—) _(ST) for the wake-up signal in node #2 is therefore:t _(WUP) _(—) _(ST) =t _(NXT)−2T _(ADD) =t _(NXT)−2θ(T ₁+(N+1)T _(PL))=t₁+(1−2θ)(T ₁+(N+1)T _(PL))   (4)

The drift clock time T_(ADD) has to be taken into account also for thecalculation of the wake-up signal duration T_(WU). If the clock in node#2 is advanced to T_(ADD) and the clock in node #1 is delayed by thesame amount, and node #2 starts the transmission of the wake-up signalWU two times T_(ADD) before t_(NXT), it could happen that the wake-upsignal WU starts 4T_(ADD) before the reception slot RX really starts innode #1. Considering this, the duration T_(WUP) of the wake-up signal WUshould be:T _(WUP) =T _(SL)+4θ(T ₁+(N+1)T _(PL))   (5)T _(WUP) =T _(SL) +T _(ADD) =T _(SL)+4θ(T ₁+(N+1)T _(PL))

As the maximum duration of signal is the listening period T_(SL), theequation (5) is modified to:T _(WUP)=mim{T _(PL) , T _(SL)+4θ(T ₁+(N+1)T _(PL))}  (6)

Thus, by calculating t_(WUP) _(—) _(ST) and T_(WUP), the duration of thewake-up signal can be reduced and hence the power consumption. Forexample, if the listening time T_(PL) is 1 s, the clock drift is 50 ppm,the reception slot time T_(SL) is 10 ms and the last communicationbetween two nodes occurred 1 minute ago, the wake-up signal lengthcalculated based on equation (6) is only 22 ms, instead of 1 s.

Additionally, to reduce the duration T_(WU) of the wake-up signal WU,every node #1 . . . #n calculates the real clock drift with any otherpeer node within its transmission range. This can be done by comparingthe values of both local and remote clocks every time a communicationhappens. The clock values can be piggybacked with the data oracknowledgements.

In each node, the clock drifts estimated with respect to each peer node#1 . . . #n are stored in a timing table in an internal memory. TheTable 2 shows an example of the timing table of node #2.

Furthermore, every node #1 . . . #n can share its table with all othernodes so that a new node joining the network does not need to learneverything about listening schedules and clock drift values from allother nodes, but use the already collected information stored in thetables of those nodes. TABLE 2 Last Time to the next Average NodeCommunication time reception slot drift Node #1 t₁ T₁ q₁ Node #3 t₃ T₃q₃ . . . . . . . . . . . . Node #n t_(n) T_(n) q_(n)

FIG. 7 shows a block diagram of a wireless node A . . . F shown inFIG. 1. As shown in FIG. 7, the node A . . . F comprises a transceiver 3which is coupled to an antenna 2 and is capable of receiving andtransmitting signals to and from the peer nodes, a controller 4 whichcontrols the transceiver 3 and calculates the timing information and amemory 5 which stores the table 2.

Although the preferred field of the invention relates to small batterypowered nodes (devices) of a wireless communication network, the presentinvention can be applied also to non-battery powered devices ofstationary communication networks advantageously.

1. A communication method between nodes in a communication system, saidcommunication system comprising at least two nodes (A, B), the methodcomprising the steps of: listening to a communication channel by adestination node (A) periodically, wherein the listen duration is T_(L)and the listen periodicity is T_(PL) and T_(L)<<T_(PL); and transmittinga wake-up signal (WU) from a source node (B) to said destination node(A) via said communication channel; characterised in that said wake-upsignal (WU) comprises a preamble (WP) and a control information (WD),wherein the duration of a signal block (SB) formed by said preamble (WP)and said control information (WD) is T_(SB) and 2×T_(SB)<T_(L); and insaid communication system the communication between nodes (A . . . F) iscontrolled based on said control information (WD).
 2. The communicationmethod according to claim 1, wherein said communication system is awireless communication system.
 3. The communication method according toclaim 1, wherein the listen duration is T_(L) and the listen periodicityis T_(PL) and 2T_(L)<<T_(PL).
 4. The communication method according toclaim 1, wherein said communication system comprises a plurality ofnodes (A . . . F) forming a network (1) and said control information(WD) indicates that said source node (B) wants to join said network (1).5. The communication method according to claim 1, wherein saidcommunication system comprises a plurality of nodes (A . . . F) forminga network (1) and said control information (WD) indicates that saidsource node (B) is still present in said network (1).
 6. Thecommunication method according to claim 1, wherein said communicationsystem comprises a plurality of nodes (A . . . F) forming a network (1)and said control information (WD) indicates an address of apredetermined node.
 7. The communication method according to claim 6,wherein, if nodes (A . . . F) receive said wake-up signal (WU) via saidcommunication channel and are not addressed by said control information(WD), said nodes (A . . . F) do not listen to said communication channeland/or transmit any data via said communication channel for apredetermined period of time.
 8. The communication method according toclaim 7, wherein said control information (WD) indicates the duration ofa subsequent data transmission and said predetermined period of time isthe duration of said subsequent data transmission time indicated by saidcontrol information (WD).
 9. The communication method according to claim1, wherein said control information (WD) indicates a time point at whichsaid source node (B) starts a data transmission and said destinationnode (A) receiving said control information (WD) starts the listening toa communication channel at said time point.
 10. The communication methodaccording to claim 9, wherein said control information (WD) indicatesthe period between the time point at which the control information istransmitted and time point at which said source node (B) starts a datatransmission.
 11. The communication method according to claim 10,wherein said wake-up signal (WU) comprises a repetition of said signalblock (SB), wherein, for each of the successive signal blocks (SB), theperiod between the time point at which the control information istransmitted and time point at which said source node (B) starts a datatransmission is calculated.
 12. The communication method according toclaim 1, wherein said control information (WD) indicates a selection ofa predetermined communication channel.
 13. The communication methodaccording to claim 1, wherein said nodes (A . . . F) interchangeinformation of listen timing (T₁, T₂) and each node accumulates saidtiming information (T₁, T₂), wherein said source node (B) calculates thestart of said wake-up signal (WU) based on said timing information (T₁,T₂).
 14. The communication method according to claim 13, wherein eachnode calculates clock drifts with respect to any other peer node basedon said timing information (T₁, T₂), wherein said source node (B)calculates the start and/or the duration of said wake-up signal (WU)based on said drift.
 15. A communication system constituted by at leasttwo nodes (A, B), the system comprising: means (3) for listening, on adestination node (A) side, to a communication channel periodically,wherein the listen duration is T_(L) and the listen periodicity isT_(PL) and T_(L)<<T_(PL); and means (3) for transmitting a wake-upsignal (WU) from a source node (B) to said destination node (A) via saidcommunication channel; characterised in that said wake-up signal (WU)comprises a preamble (WP) and a control information (WD), wherein theduration of a signal block (SB) formed by said preamble (WP) and saidcontrol information (WD) is T_(SB) and 2×T_(SB)<T_(L); and in saidcommunication system the communication between nodes (A . . . F) iscontrolled based on said control information (WD).
 16. The communicationsystem according to claim 15, wherein said communication system is awireless communication system.
 17. The communication system according toclaim 15, wherein the listen duration is T_(L) and the listenperiodicity is T_(PL) and 2T_(L)<<T_(PL).
 18. The communication systemaccording to claim 15, wherein said communication system comprises aplurality of nodes (A . . . F) forming a network (1) and saiddestination node (A) is adapted to detect whether said controlinformation (WD) indicates that said source node (B) wants to join saidnetwork (1).
 19. The communication system according to claim 15, whereinsaid communication system comprises a plurality of nodes (A . . . F)forming a network (1) and said destination node (A) is adapted to detectwhether said control information (WD) indicates that said source node(B) is still present in said network (1).
 20. The communication systemaccording to claim 15, wherein said communication system comprises aplurality of nodes (A . . . F) forming a network (1) and saiddestination node (A) is adapted to detect whether said controlinformation (WD) indicates an address of a predetermined node.
 21. Thecommunication system according to claim 20, wherein said destinationnode (A) is adapted to stop the listening to said communication channeland/or the transmission of any data via said communication channel for apredetermined period of time, if said destination node (A) is notaddressed by said control information (WD).
 22. The communication systemaccording to claim 21, wherein said destination node (A) is adapted todetect whether said control information (WD) indicates the duration of asubsequent data transmission and said predetermined period of time isthe duration of said subsequent data transmission time indicated by saidcontrol information (WD).
 23. The communication system according toclaim 15, wherein said destination node (A) is adapted to detect whethersaid control information (WD) indicates a time point at which saidsource node (B) starts a data transmission and to listen to acommunication channel at said time point.
 24. The communication systemaccording to claim 23, wherein said control information (WD) indicatesthe period between the time point at which the control information istransmitted and time point at which said source node (B) starts a datatransmission.
 25. The communication system according to claim 24,wherein said source node (B) is adapted to generate a wake-up signal(WU) comprising a repetition of said signal block (SB), wherein saidsource node (B) comprises means for calculating, for each of thesuccessive signal blocks (SB) in said wake-up signal (WU), the periodbetween the time point at which the control information is transmittedand the time point at which said source node (B) starts a datatransmission.
 26. The communication system according to claim 15,wherein said destination node (A) is adapted to detect whether saidcontrol information (WD) indicates a predetermined communicationchannel.
 27. The communication system according to claim 15, whereinsaid nodes (A . . . F) are adapted to interchange information of listentiming (T₁, T₂) and each node is adapted to accumulate said timinginformation (T₁, T₂), wherein said source node (B) is adapted tocalculate the start of said wake-up signal (WU) based on said timinginformation (T₁, T₂).
 28. The communication system according to claim27, wherein each node is adapted to calculate clock drifts with respectto any other peer node based on said timing information (T₁, T₂),wherein said source node (B) is adapted to calculate the start and/orthe duration of said wake-up signal (WU) based on said drift. 29.Apparatus for requesting services from a remote apparatus, said remoteapparatus (A) listens to a communication channel periodically, whereinthe listen duration is T_(L) and the listen periodicity is T_(PL) andT_(L)<<T_(PL), the requesting apparatus (B) comprising: wake-up signal(WU) generating means for generating a wake-up signal (WU); controlmeans (4) for controlling said wake-up signal generating means togenerate said wake-up signal (WU), if a service has to be requested fromsaid remote apparatus; and transmitting means (3) for transmitting thewake-up signal (WU) generated by said wake-up signal (WU) generatingmeans to said remote apparatus via said communication channel,characterised in that said wake-up signal (WU) comprises a preamble (WP)and a control information (WD), wherein the duration of the signal block(SB) formed by said preamble (WP) and said control information (WD) isT_(SB) and 2×T_(B)<T_(L).
 30. The apparatus according to claim 29,wherein the listen duration is T_(L) and the listen periodicity isT_(PL) and 2T_(L)<<T_(PL).
 31. The apparatus according to claim 29,wherein said wake-up signal generating means is adapted to add a controlinformation (WD) including a network (1) join request to said wake-upsignal (WU).
 32. The apparatus according to claim 29, wherein saidwake-up signal generating means is adapted to add a control information(WD) including a presence notice to said wake-up signal (WU).
 33. Theapparatus according to claim 29, wherein said wake-up signal generatingmeans is adapted to add a control information (WD) indicating theaddress of a remote apparatus from which a service has to be requestedto said wake-up signal (WU).
 34. The apparatus according to claim 29,wherein said wake-up signal generating means is adapted to add a controlinformation (WD) indicating the duration of a subsequent datatransmission to said wake-up signal (WU).
 35. The apparatus according toclaim 29, wherein said wake-up signal (WU) generating means is adaptedto add a control information (WD) indicating a time point at which thetransmitting means (3) starts a data transmission to said wake-up signal(WU).
 36. The apparatus according to claim 35, wherein said controlinformation (WD) indicates the period between the time point at whichthe control information is transmitted and time point at which said thetransmitting means (3) starts a data transmission.
 37. The apparatusaccording to claim 36, wherein said wake-up signal (WU) generating meansis adapted to generate a wake-up signal (WU) comprising a repetition ofsaid signal block (SB), wherein said wake-up signal (WU) generatingmeans comprises means for calculating, for each of the successive signalblocks (SB) in said wake-up signal (WU), the period between the timepoint at which the control information is transmitted by saidtransmitting means (3) and the time point at which said transmittingmeans (3) starts a data transmission.
 38. The apparatus according toclaim 29, wherein said wake-up signal generating means is adapted to adda control information (WD) indicating a predetermined communicationchannel to said wake-up signal (WU).
 39. The apparatus according toclaim 29, further comprising means for receiving information of listentiming (T₁, T₂) from a remote apparatus, wherein said control means (4)is adapted to calculate the start of said wake-up signal (WU) based onsaid timing information (T₁, T₂).
 40. The apparatus according to claim39, wherein said control means (4) is adapted to calculate clock driftswith respect to said remote apparatus based on said timing information(T₁, T₂) and, wherein said control means (4) is adapted to calculate thestart and/or the duration of said wake-up signal (WU) based on saiddrift.
 41. Apparatus for providing services requested from a remoteapparatus, the apparatus comprising: receiving means (3) for receiving awake-up signal (WU) send from said remote apparatus via a communicationchannel; control means (4) for controlling said receiving means (3) tolisten to said communication channel periodically, wherein the listenduration is T_(L) and the listen periodicity is T_(PL) andT_(L)<<T_(PL), and for providing a service, if said wake-up signal (WU)is received by said receiving means (3), characterised in that saidwake-up signal (WU) comprises a preamble (WP) and a control information(WD), wherein the duration of the signal block (SB) formed by saidpreamble (WP) and said control information (WD) is T_(SB) and2×T_(SB)<T_(L); a separating means separates said control information(WD) from said wake-up signal (WU); and said control means (4) controlssaid receiving means (3) based on said control data separated by saidseparating means.
 42. The apparatus according to claim 41, wherein thelisten duration is T_(L) and the listen periodicity is T_(PL) and2T_(L)<<T_(PL).
 43. The apparatus according to claim 41, wherein saidcontrol means (4) is adapted to detect whether said control information(WD) includes a network (1) join request.
 44. The apparatus according toclaim 41, wherein said control means (4) is adapted to detect whethersaid control information (WD) includes a presence notice.
 45. Theapparatus according to claim 41, wherein said control means (4) isadapted to detect whether said control information (WD) includes anapparatus address.
 46. The apparatus according to claim 45, wherein saidcontrol means (4) is adapted to control said receiving means (3) to stopthe listening to said communication channel and/or the transmission ofany data via said communication channel for a predetermined period oftime, if said service providing apparatus is not addressed by saidcontrol information (WD).
 47. The apparatus according to claim 46,wherein said control means (4) is adapted to detect whether said controlinformation (WD) indicates the duration of a subsequent datatransmission of said remote apparatus and wherein, said predeterminedperiod of time is the duration of said subsequent data transmission timeindicated by said control information (WD).
 48. The apparatus accordingto claim 41, wherein said control means (4) is adapted to detect whethersaid control information (WD) indicates a time point at which saidremote apparatus starts a data transmission and to control saidreceiving means (3) to listen to a communication channel at said timepoint.
 49. The apparatus according to claim 48, wherein said controlinformation (WD) indicates the period between the time point at whichthe control information is transmitted and time point at which said thetransmitting means (3) starts a data transmission.
 50. The apparatusaccording to claim 41, wherein said control means (4) is adapted todetect whether said control information (WD) indicates a selection of apredetermined communication channel.
 51. The apparatus according toclaim 41, wherein said control means (4) is adapted to detectinformation of listen timing (T₁, T₂) of a remote apparatus received bysaid receiving means (3), wherein said control means (4) is adapted tocalculate the start of a wake-up signal (WU) based on said timinginformation (T₁, T₂).
 52. The apparatus according to claim 51, whereinsaid control means (4) is adapted to calculate clock drifts with respectto said remote apparatus based on said timing information (T₁, T₂) and,wherein said control means (4) is adapted to calculate the start and/orthe duration of a wake-up signal (WU) based on said drift.